Magnetic recording medium, method for producing the same and method for forming film by plasma CVD

Abstract: A magnetic recording medium comprising a back coating layer formed on a surface reverse to a magnetic layer and a carbonaceous film which is formed on the back coating layer and contains fluorine atoms and silicon atoms and/or nitrogen atoms and in which a concentration of fluorine atoms decreases in a depth direction from a surface of the carbonaceous film, while a concentration of the silicon and/or nitrogen atoms increases in the depth direction from the surface of the carbonaceous film, which recording medium has improved running stability, durability and weatherability while electromagnetic conversion characteristics are maintained.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for forming a film by a plasma CVD (chemical vapor deposition). In particular, the present invention relates to a magnetic recording medium which is used in an audio or video equipment, a computer, and the like, and has a back coating layer on a surface of a nonmagnetic substrate reverse to a magnetic recording layer. Description of the Related Art In these years, a magnetic recording equipment is required to have a large capacity, achieve high speed reading and writing, high picture and sound quality, and have a small size and a light weight. To satisfy such requirements, it is inevitable for a magnetic recording medium to achieve a high density recording. To this end, instead of a conventional powder coating type magnetic recording medium having a magnetic layer which comprises a binder and magnetic powder dispersed in said binder, a ferromagnetic metal thin film magnetic recording medium has been actively developed and practically used, since it has a larger residual magnetic flux density (Br) and coercive force (Hc) and a thinner magnetic layer, and is more suitable for ultra-smoothing of a magnetic layer surface than the powder coating type one. With the increase of surface smoothness of the magnetic layer, a coefficient of friction of the magnetic recording medium increases, so that running stability and durability tend to be worsened. Then, it is proposed to provide a back coating layer having specific surface roughness on a surface of a nonmagnetic substrate reverse to the magnetic layer. The surface roughness of the back coating layer, in particular that of the ferromagnetic metal thin film magnetic recording medium should be as small as possible to decrease a so-called back transfer, that is, shape transfer of unevenness of the back coating layer to the magnetic layer surface in a winding up step or a heat treating step, and to prevent deterioration of electromagnetic conversion characteristics. To this end, it is proposed to decrease a particle size of a filler to be contained in the back coating layer, or to add an excessive amount of a dispersant or use a binder having a relatively large amount of a polar group such as a sulfonate in a molecule for improving dispersibility of the filler. However, these measures result in decrease of film strength of the back coating layer, so that the back coating layer tends to be scraped off or a recording tape tends to be folded during tape running. In addition, worn debris from the back coating layer is transferred to the magnetic layer surface during tape winding and may clog a magnetic head, increase the number of drop-outs or decrease output signal level. When the magnetic recording medium is stored for a long time, it suffers from blocking, namely adhesion of the magnetic layer and the back coating layer each other. Since the magnetic layer of the ferromagnetic metal thin film magnetic recording medium has low hardness and is easily abraded, it is proposed to form, on the magnetic layer, a diamond-like carbon film and a fluorine-containing lubricant layer having a lubricity and water-repellency, successively to improve running stability and durability. However, since a surface state of the diamond-like carbon film is very inactive chemically, when it is stored in a high temperature high humidity atmosphere, the lubricant component migrates to the back coating layer so that an amount of the lubricant on the diamond-like carbon film is reduced, and then a still-frame life is decreased. JP-A-353616/1992 discloses a magnetic recording medium comprising a back coating layer on a surface of a nonmagnetic substrate reverse to a magnetic layer and a fluorine-containing hard carbon film on the back coating layer to improve wear resistance of the back coating layer and also to prevent the blocking and migration of a lubricant component from a lubricant layer to the back coating layer. However, it is still difficult to provide a magnetic recording medium which is excellent both in the electromagnetic conversion characteristics and durability. There are still many problems to be solved. In the structure in which the fluorine-containing hard carbon film is formed directly on the back coating layer, while a bond energy between a fluorine atom and a carbon atom in the hard carbon film is very large, an interaction between the fluorine atom and an atom on the back coating layer surface such as a carbon atom or an oxygen atom is very weak, and therefore, the fluorine-containing hard carbon film is peeled off at an interface between the hard carbon film and the back coating layer during tape running or winding, and peeled-off particles are transferred on the magnetic layer surface to cause the significant decrease of output signal or long-time clogging of the magnetic head. Further, when a content of the fluorine atoms in the hard carbon film is large, hardness of the carbon film decreases so that the running stability of the magnetic recording medium is deteriorated. EP-A-0264699, representing the closed prior art for which the invention proceeds, discloses a method for depositing protective coatings on magnetic tapes provided with metal film wherein the top of a partition wall terminates very closely to a temperature-adjusted drum so that there is only a very thin gap between the partition wall and the drum. The film formed by such method includes two distinct layers, the first one of which is formed in a first vacuum system at the one side of the partition wall and the second one of which is formed in a second vacuum system at the other side of the partition wall. As a result, the film cannot have a continuously changed composition. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved method for forming a film by a plasma CVD method. In order to achieve the above and further objects, according to the present invention, there is provided a method for forming a film having continuously changed composition by a plasma CVD method comprising generating glow discharge plasma in a substrate placed in a vacuum chamber using a discharge tube facing said substrate and having at least one partition plate, characterized in that a distance between a top end of said partition plate and said substrate surface is larger than a distance between a top end of said discharge tube and said substrate surface. According to a preferred embodiment of the present invention, the distance between the top end of said partition plate and said substrate surface is at least 2 times larger than the distance between the top end of said discharge tube and said substrate surface. After all, a magnetic recording medium produced by the method according to the present invention can achieve good wear resistance and low energy surface properties (water and oil repellency) while maintaining the good adhesion between the carbonaceous film and the back coating layer. Therefore, the present invention can provide a magnetic recording medium which is excellent in the running stability, durability and weatherability without deteriorating the electromagnetic conversion characteristics. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged cross sectional view of an embodiment of a ferromagnetic metal thin film magnetic tape according to the present invention, and Fig. 2 schematically shows an example of an apparatus for forming a carbonaceous film constituting a ferromagnetic metal thin film magnetic tape according to the present invention by the plasma CVD method. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The present invention will be explained in detail by making reference to the accompanying drawings. Fig. 1 shows an enlarged cross section of an example of the ferromagnetic metal thin film magnetic tape according to the present invention, which comprises a nonmagnetic substrate 1, on one surface, a ferromagnetic metal thin film 3, a diamond-like carbon film 4, a lubricant layer 7, and on the other surface, a back coating layer 5 and a carbonaceous film 6. The nonmagnetic substrate may be made of any one of conventionally used materials, preferably, a polymer film of, for example, polyethylene terephthalate, polyethylene naphthalate, polyamide, polyimide, and so on. A thickness of the nonmagnetic substrate is not critical and can be the same as that of the conventional nonmagnetic substrate. If desired, a minute protrusion layer 2 is formed on the surface of the nonmagnetic substrate on which the magnetic layer is formed. Preferably, the minute protrusion layer has a maximum height roughness (R max ) of 10 nm to 20 nm. The ferromagnetic metal thin film 3 may be formed by a conventional method. For example, a ferromagnetic metal or an alloy of ferromagnetic metals such as Co, Co-Ni, etc. is heated and evaporated by, for example, an electron beam under reduced pressure in a vacuum chamber and deposited on the substrate by an oblique vapor deposition method in which an incident angle of the metal vapor is continuously changed while introducing a small amount of an oxygen gas in the vacuum chamber. A thickness of the ferromagnetic metal film 3 is preferably from 150 to 200 nm, while it is possible to decrease or increase the thickness depending on a kind of the magnetic recording medium. The diamond-like carbon film 4 can be formed by any one of conventional methods such as a plasma CVD method, an ion beam vapor deposition method, an ion beam sputtering method, a laser vapor deposition method, and so on. A thickness of the diamond-like film 4 is arbitrary and preferably from 8 nm to 12 nm to maintain a reproducing signal level in a short wavelength range. The back coating layer 5 is formed by, for example, coating a back coating paint comprising a filler (e.g. carbon black, calcium carbonate, etc.), a binder resin (e.g. polyester resin, nitrocellulose resin, etc.) and other necessary additive or additives and drying it. A thickness of the back coating layer is not critical and preferably up to about 500 nm. The carbonaceous film 6 contains the fluorine atoms and at least one kind of atoms selected from the group consisting of silicon atoms and nitrogen atoms. In this film 6, a concentration of the fluorine atoms decreases in the depth direction from the surface of the film 6, while a concentration of the silicon and/or nitrogen atoms increases in the depth direction from the surface of the film 6. The concentration of the fluorine atoms may change continuously or stepwise. A thickness of the carbonaceous film 6 is usually from 3 nm to 50 nm, preferably from 5 nm to 30 nm. The lubricant layer 7 preferably comprises a fluorine-containing lubricant and is formed by a wet coating method or an organic vapor deposition method. Its thickness is usually up to about 3 nm. The lubricant preferably has at least one polar group, such as -COOH, -OH, -SH, -NH 2 , >NH, -NCO, -CONH 2 , -CONHR, -CONR 2 , -COOR, >PR, >PRO, -PRS, -OPO(OH) 2 , -OPO(OR) 2 , and -SO 3 M in which R is a C 1 -C 22 hydrocarbon group, and M is a hydrogen atom, an alkali metal or an alkaline earth metal. Fig. 2 schematically shows an embodiment of an apparatus for forming the carbonaceous film 6 constituting the ferromagnetic metal thin film magnetic tape according to the present invention by the plasma CVD method. The apparatus comprises a vacuum chamber 8 which is evacuated by a vacuum pump 9 to a pressure of, for example, 10 -4 torr to 10 -5 torr. In the chamber 8, a sheet 10 for a magnetic recording tape which comprises the nonmagnetic substrate 1, the ferromagnetic metal thin film 3, the back coating layer 5 and the diamond-like carbon film 4 is supplied from an unwinder roll 11, passes around a pass roll 12, a cylindrical cooling can 14 and then a pass roll 13, and is taken up on a winder roll 15. The cooling can 14 functions to control a conveying rate of the sheet 10 at a constant rate. In the vacuum chamber 8, there is a discharge tube 28 for forming the carbonaceous film 6 on the back coating layer of the sheet 10. The discharge tube 28 has a partition plate 29 and two pipe-form discharge electrodes 30 and 31. In this embodiment, a distance between the top end of the discharge tube 28 and the sheet 10 is 0.5 mm, and a distance between the top end of the partition plate 29 and the sheet 10 is 5 mm. The pipe-form discharge electrodes 30 and 31 are connected to plasma-generating power sources 32 and 33, respectively. As the power source, any of a system for applying a direct current or an alternating current and a system for superposing the direct current and the alternating current can be used. The fluorine-containing organic compound gas, the silicon-containing organic compound gas, the nitrogen-containing organic compound gas, the silicon and nitrogen-containing organic compound gas, and an inert gas such as argon can be supplied in the discharging electrode 28 from gas inlets 34 and 35. As the fluorine-containing organic compound, the silicon-containing organic compound, the nitrogen-containing organic compound, or the silicon and nitrogen-containing organic compound, any of conventionally used organic compounds may be used. Preferred examples of the fluorine-containing organic compound are fluoromethane, difluoromethane, trifluoromethane, tetrafluoromethane, fluoroethylene, difluoroethylene, trifluoroethylene, tetrafluoroethylene, fluoroethane, difluoroethane, trifluoroethane, tetrafluoroethane, pentafluoroethane, hexafluoroethane, fluoropropylene, difluoropropylene, trifluoropropylene, tetrafluoropropylene, pentafluoropropylene, hexafluoropropylene, fluorobutadiene, difluorobutadiene, trifluorobutadiene, tetrafluorobutadiene, pentafluorobutadiene, hexafluorobutadiene, octafluorocyclobutane, perfluorobenzene, perfluorotoluene, and the like. Preferred examples of the silicon-containing organic compound are trimethoxysilane, tetramethylsilane, methoxytrimethylsilane, dimethoxydimethylsilane, diethoxysilane, methyltrimethoxysilane, tetramethoxysilane, allyldimethylsilane,trimethylvinylsilane, ethyltrimethylsilane, ethyltrimethoxysilane, tetramethyldisiloxane, pentamethyldisiloxane, hexamethyldisiloxane, and the like. Preferred examples of the nitrogen-containing organic compound are methylamine, ethylamine, propylamine, butylamine, pentylamine, vinylamine, allylamine, dimethylamine, diethylamine, trimethylamine, triethylamine, pyridine, picoline, pyridazine, pyrimidine, pyrazine, acrylamide, acrylonitrile, acetonitrile, aniline, toluidine, phenylenediamine, and the like. Preferred examples of the silicon and nitrogen-containing organic compound are aminomethyltrimethylsilane, dimethyldimethylaminosilane, dimethylaminotrimethylsilane, tetramethyldisilazane, hexamethyldisilazane, heptamethyldisilazane, hexamethoxydisilazane, and the like. In addition to the above organic compounds, a hydrocarbon which does not contain fluorine, silicon or nitrogen atoms such as methane, ethane, benzene, toluene and so on can be used. The present invention will be illustrated by the following examples, which do not limit the scope of the invention. Example 1 On one surface of a polyethylene terephthalate film 1 having a thickness of 10 µm which surface had a minute protrusion layer 2 having the maximum height roughness R max of about 15 nm measured by the surface structure analysis with a scanning tunneling microscope (STM), a ferromagnetic metal thin film 3 of Co-O having a thickness of 180 nm was formed by the continuously incident angle changing vapor deposition method. On the ferromagnetic metal thin film 3, a diamond-like carbon film 4 having a thickness of 10 nm was formed by the plasma CVD method using a mixed gas of methane and argon in a volume ratio of 4:1. Further, on the other surface of the polyethylene terephthalate film 1, a back coating layer 5 having a thickness of 500 nm after drying was formed by coating the same solution as used in Example 1 by the wet coating method. Then, the formed sheet 10 was set on the supply roll 11 in the vacuum chamber 8 of the apparatus of Fig. 2. After evacuating the interior of the vacuum chamber 8, from the inlet 34, tetrametyldisiloxane and argon in a volume ratio of 4:1 were introduced in the discharge tube 28 with adjusting a total gas pressure to 27 Pa (0.20 torr.) by controlling the flow rates of the gasses. From the inlet 35, octafluorocyclobutane and argon in a volume ratio of 4:1 were introduced in the discharge tube 28 with adjusting a total gas pressure to 27 Pa (0.20 torr.) by controlling the flow rates of the gasses. Thereafter, the sheet 10 was conveyed at a running rate of 15 m/min. around the can 14 while a direct current of 800 V and an alternating current of 800 V having a frequency of 20 kHz were superposed and applied to each of the discharge electrodes 30 and 31 to generate nonequilibrium plasma in the discharge tube 28, whereby a carbonaceous film 6 having a thickness of 10 nm was formed on the surface of the back coating layer 5, in which film 6, a concentration of the fluorine atoms continuously decreased in the depth direction from its surface towards the interface between the back coating layer and the carbonaceous film, while a concentration of the silicon atom increased in the depth direction from its surface. Further, on the diamond-like carbon film 4, a solution of C 5 F 11 (CH 2 ) 10 COOH in isopropanol at a concentration of 2000 ppm was wet coated and dried to form a lubricant layer 7 having a thickness of 3 nm. Then, the sheet was slit to a width of 8 mm to obtain a 8 mm VTR tape. Before the formation of the lubricant layer 7, the chemical composition of the carbonaceous film 6 on the sheet 10 was analyzed by X-ray photoelectron spectroscopy (XPS) to find that an atomic ratio of the fluorine atoms to the carbon atoms near the surface of the carbonaceous film 6 was 23 %. A critical surface tension γ c on the surface of the carbonaceous film 6 of the magnetic tape was 18.6 x 10 -5 N/cm. Examples 2 In the same manner as in Example 1 except that pyridine was used in place of tetramethyldisiloxane, pyridine and argon in a volume ratio of 4:1 were introduced in the discharge tube 28 from the inlet 34, the total gas pressure in the discharge tube 28 was adjusted to 33 Pa (0.25 torr.) and a direct current of 1000 V and an alternating current of 1300 V having a frequency of 20 kHz were superposed and applied to the discharge electrode 30, and a thickness of the carbonaceous film 6 was 21 nm, a 8 mm VTR tape was produced. Before the formation of the lubricant layer 7, the chemical composition of the carbonaceous film 6 on the sheet 10 was analyzed by X-ray photoelectron spectroscopy (XPS) to find that an atomic ratio of the fluorine atoms to the carbon atoms near the surface of the carbonaceous film 6 was 20 %. A critical surface tension γ c on the surface of the carbonaceous film 6 of the magnetic tape was 18.9 x 10 -5 N/cm. In the Examples the critical surface tension γ c on the surface of the carbonaceous film 6 of the magnetic tape was obtained by the following method: Each of four wettability standard solutions having surface tensions of 38 x 10 -5 , 45 x 10 -5 , 54 x 10 -5 and 72 x 10 -5 N/cm, respectively, was dropped on the surface of the carbonaceous film and a contact angle  of each of the solutions was measured.. Then, cosines of angles  (cos) were plotted against surface tensions of the wettability standard solutions (Zisman plot). A surface tension corresponding to an intersection between a line obtained by the least square method from the plot and the line at cos = 1.0 was read and this value was defined as the critical surface tension γ c . With each of the VTR tapes produced in the Examples, the following properties were measured: (1) Coefficient of friction µ k Around a quarter of a periphery of a stainless steel (SUS 420J2) cylinder having a diameter of 4 mm and a surface roughness of 0.2 S, the tape was wound with contacting the back coating layer to the cylinder surface. Then, the tape is slid at a rate of 0.5 mm/sec. under a tension of 30 g at one end, and a tension X g at the other end is measured at a temperature of 25°C and a humidity of 30 %RH. A coefficient of friction is calculated according to the following equation: µ k = (2/π)ln(X/30) A coefficient of friction obtained after 30 passes is used. (2) Decrease of output signal At 5°C, 80 %RH, a video signal is recorded on a magnetic tape of about 60 minutes long, and reproduced in 200 passes (the running durability test). With using the RF output in the first pass as the standard (0 dB), the lowest RF output in the 200 passes is represented in the unit of "dB". (3) Head clogging In the above running durability test, a time at which the RF output decreases by 6 dB or larger continuously is measured, and the total time is defined as a head clogging time. (4) Anti-blocking At 40°C, 90 %RH, a magnetic tape is stored for about 30 days, and generation of blocking is observed (O: No blocking, X: Blocked). (5) Still-frame life A magnetic tape which has been stored at 40°C, 90 %RH for about 10 days is reproduced in a still mode at 23°C, 10 %RH under a load of 20 g, and a time at which the RF output decreases by 6 dB is measured. The measurement is terminated after 60 minutes. The results of the above tests are shown in the Table. Example No. Coefficient of friction µk Decrease of output signal (dB) Head clogging (sec/200 hr) Antiblocking Stillframe life (min.) Exam. 1 0.14 0 0 O >60 Exam. 2 0.15 0 1 O >60 As seen from the results in the Table, the carbonaceous films produced in the above Examples contained the fluorine atoms and the silicon and/or nitrogen atoms and the concentration of the fluorine atoms decreased in the depth direction from the surface of the carbonaceous film while the concentrations of the silicon and/or nitrogen atoms increased in the depth direction from the surface of the carbonaceous film. Since such carbonaceous film was formed on the back coating layer, the adhesion between the carbonaceous film and the back coating layer was maintained good and further the carbonaceous film could achieve sufficient wear resistance, low energy surface properties (water and oil repellency) and so on. Accordingly, the running stability, decrease of output signal and head clogging were greatly improved, and also anti-blocking and still-frame life were increased. In particular, since the concentrations of the fluorine atoms and the silicon or nitrogen atoms in the carbonaceous film changed continuously, internal stress in the carbonaceous film was relaxed and then the adhesion with the back coating layer was further increased, whereby the practical reliability such as running stability, durability and weatherability was considerably improved. In the above Examples, the metal thin film 8 mm VTR tapes were produced. The present invention can be applied to other metal thin film magnetic recording media, and the particulate magnetic recording media. The plasma CVD method which can form the carbonaceous film having a continuously changed composition according to the present invention can be employed in the formation of the plasma polymerization film or a diamond-like carbon film which is used as a protective film on a ferromagnetic metal thin film, the formation of an insulating film for a semiconductor or a liquid crystal, the formation of an amorphous silicon which is used in an photoelectric transfer device, the formation of a superconductive thin film, and so on.

Claims: A method for forming a film having a continuously changed composition by a plasma CVD method comprising generating glow discharge plasma on a substrate (10) placed in a vacuum chamber (8) using a discharge tube (28) facing said substrate and having at least one partition plate (29), wherein said substrate is moved past said discharge tube (28) and a different source gas composition is supplied to each side of said partition plate (29); characterized in that a distance between a top end of said partition plate (29) and said substrate surface is larger than a distance between a top end of said discharge tube (28) and said substrate surface. The method according to claim 1, wherein the distance between the top end of said partition plate (29) and said substrate surface is at least 2 times larger than the distance between the top end of said discharge tube (28) and said substrate surface.

Inventors: KUWAHARA KENJI, MURAI MIKIO, ODAGIRI MASARU, SEKI HIROSHI, TAKAHASI KIYOSI, UEDA HIDEYUKI, KUWAHARA, KENJI, MURAI, MIKIO, ODAGIRI, MASARU, SEKI, HIROSHI, TAKAHASI, KIYOSI, UEDA, HIDEYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: G11B5
IPC codes: G11B5